1. Field of the Invention
The present invention relates to a heat treatment processing apparatus and a cleaning method thereof, in particular, to a heat treatment apparatus for diffusing impurities in a workpiece such as a semiconductor wafer and a cleaning method thereof.
2. Description of the Related Art
In a fabrication process of a semiconductor device, a thin film, an oxide film, or the like is deposited over a semiconductor wafer or the like. When impurities are diffused in a semiconductor wafer, a CVD apparatus, an oxide film forming apparatus, a diffusing apparatus or the like is used. These apparatuses have a heat treatment apparatus. The heat treatment apparatus has a reaction vessel in which many wafers are simultaneously processed with a reaction gas which is heated at a high temperature. Thus, the heat treatment processing apparatus has been widely used in the field of the semiconductor fabrication. However, the heat treatment apparatus requires a flow rate control unit which properly controls the flow rate of the reaction gas supplied to the reaction vessel. As an example of the flow rate control unit, a mass flow controller which has a gas flow rate detecting sensor and a flow rate controlling means which closes and opens a passageway of a reaction gas corresponding to a value detected by the gas flow rate detecting sensor.
The heat treatment processing apparatus is provided with a final filter disposed in the middle of a reaction gas supply pipe connected between the reaction vessel and the mass flow controller. The final filter captures particles contained in the reaction gas. A liquified substance of the reaction gas and the like adhere to the filter. Thus, the filter tends to clog. In addition, the liquified substance of the reaction gas and the like adhere to the orifice portion of the mass flow controller. Thus, the mass flow controller also tends to clog. Therefore, the interior of the reaction gas supply pipe should be periodically deaerated so as to clean the liquefied substance of the reaction gas and the like which adhere to the filter. However, if an inflammable gas or a poisonous gas is used as the reaction gas, before the interior of the reaction gas supply pipe is deaerated, an inert gas such as nitrogen gas N.sub.2 should be supplied into the reaction gas supply pipe so as to substitute the reaction gas with the inert gas. Thus, the heat treatment processing apparatus has an inert gas supply source and a vacuum pump. The inert gas supply source and the vacuum pump are disposed on an upstream side of the mass flow controller and on a downstream side thereof, respectively. The inert gas supply pipe and the vacuum exhaust pipe each have a valve. In addition, the reaction gas supply pipe has valves on a reaction vessel side (downstream side) and a reaction gas supply source side (upstream side). By properly opening and/or closing these valves, the reaction gas is supplied to the reaction vessel, the interior of the reaction gas supply pipe is deaerated, and the inert gas is supplied to the reaction gas supply pipe.
In other words, when wafers are processed, both the valves on the upstream side and the downstream side of the reaction gas supply pipe are opened. In addition, the valves on the inert gas supply pipe and the vacuum exhaust pipe are closed. In this condition, the reaction gas is supplied from the reaction gas supply source to the reaction vessel through the filter, the mass flow controller, and the final filter so as to process the semiconductor wafers. At this point, the reaction vessel is deaerated to a predetermined degree of vacuum by the vacuum pump disposed on a downstream side thereof. The waste gas is sent to a gas processing unit through a vacuum pump.
On the other hand, when the interior of the reaction gas supply pipe is deaerated and cleaned, the valve on the upstream side of the reaction gas supply pipe and the valve for the vacuum exhaust pipe are closed and the valve for the inert gas supply pipe is opened so as to supply the inert gas from the inert gas supply source to the reaction gas supply pipe and substitute the reaction gas with the inert gas. This gas substitution is performed by the inert gas supplied from the upstream side (primary side). After the reaction gas in the reaction gas supply pipe is substituted with the inert gas, the valve for the inert gas supply pipe is closed and the valve for the vacuum exhaust pipe is opened so as to deaerate the interior of the reaction gas supply pipe both from the vacuum exhaust-pipe side and the reaction vessel side. By performing the deaerating process, a liquified substance of the reaction gas and the like which adhere to the final filter are removed. After this deaerating process is properly performed, the valve for the vacuum exhaust pipe is closed so as to complete the cleaning process. After the cleaning operation is completed, new wafers, which have not been processed, are set in the reaction vessel. By opening the valve on the primary side of the reaction gas supply pipe, the reaction gas is supplied to the reaction vessel so as to process these wafers.
Thus, since the interior of the reaction gas supply pipe is deaerated and cleaned before new wafers are processed, the final filter and the mass flow controller can be prevented from clogging. In addition, when the wafers are processed, the reaction gas can be always supplied at a proper flow rate.
However, in the above-mentioned cleaning method, since the inert gas is supplied from the upstream side of the apparatus through the narrow orifice portion of the mass flow controller, the gas substituting efficiency is low. In addition, when the interior of the reaction gas supply pipe is deaerated from the downstream side thereof, the orifice portion of the mass flow controller disturbs the deaerating process. Thus, the interior of the reaction gas supply pipe is not quickly deaerated, but gradually. Therefore, the cleaning process for the interior of the reaction gas supply pipe takes a long time.
To prevent this problem, a bypass pipe which has an open/close valve and detours around the mass flow controller is disposed in the middle of the reaction gas supply pipe. When the reaction gas is supplied, the open/close valve for the bypass pipe is closed. When the gas substituting process is performed, this valve is opened so as to supply the gas. Thus, the inert gas can be smoothly supplied to the downstream side of the mass flow controller. However, the deaerating efficiency is still low. In addition, the reaction gas which resides in the bypass pipe tends to corrode it.